Water Breakup at Fe₂O₃–Hematite/Water Interfaces: Influence of External Electric Fields from Nonequilibrium Ab Initio Molecular Dynamics

Abstract

The dynamical properties of physically and chemically adsorbed water molecules at pristine hematite-(001) surfaces have been studied by means of nonequilibrium ab initio molecular dynamics (NE-AIMD) in the NVT ensemble at room temperature, in the presence of externally applied, uniform static electric fields of increasing intensity. The dissociation of water molecules to form chemically adsorbed species was scrutinized, in addition to charge redistribution and Grotthus proton hopping between water molecules. Dynamical properties of the adsorbed water molecules and OH – and H 3 O + ions were gauged, such as the hydrogen bonds between protons in water molecules and the bridging oxygen atoms at the hematite surface, as well as the interactions between oxygen atoms in adsorbed water molecules and iron atoms at the hematite surface. The development of Helmholtz charge layers via water breakup at Fe 2 O 3 –hematite/water interfaces is also an interesting feature, with the development of protonic conduction on the surface and more bulk-like water.

Electric Field Effects on Photoelectrochemical Water Splitting: Perspectives and Outlook

“…An important insight of ref. [75] on the experiments of refs. [17,74] lies in now the externally applied electric field alter the actual intrinsic field and the underlying electrostatic potential themselves in photo-absorbing materials: here, non-equilibrium ab initio MD can be very informative.…”

“…Although we have outlined interesting seminal work on electric field effects in PEC [60][61][62], the first actual reported disclosure on electric field boosting of PEC performance, whether arising from intrinsic or externally applied fields, was a patent filed by English describing both experimental and molecular simulation invention findings [74], with further and wider ranging details relating thereto (beyond the core aspects of the invention of field manipulation of solar activity), which he later published in a journal [75]. We shall discuss the experimental and simulation aspects of refs.…”

“…This is greater than a DC field (i.e., "zero" frequency in Figure 6), where dipole alignment is maintained, lessening comparative scope for intramolecular strain [17]; this 2-ps period in the 500-GHz case is nearer to usual individual dipole relaxation times [17], providing good opportunities for enhanced extent of aligning. Further, Futera and English performed NE-AIMD simulations on DC electric fields applied to similar hematite Fe2O3 (001)/water interfaces [75]. The break-up of water molecules to form chemisorbed species was measured, as well as uncovering charge redistribution and Grotthus proton hopping between water molecules [75].…”

“…[17,74] lies in now the externally applied electric field alter the actual intrinsic field and the underlying electrostatic potential themselves in photo-absorbing materials: here, non-equilibrium ab initio MD can be very informative. Futera and English determined the electronic density and total charge density profiles, as well as how the Hartree potential varies in space at hematite/water interfaces [75]. They determined how these quantities can show a substantial drop at the interface, which shows how the fields manipulate the mean inner potential of both the interfacial water and of the hematite itself, to further render the hematite more susceptible towards charge-carrier separation [75].…”

“…Futera and English determined the electronic density and total charge density profiles, as well as how the Hartree potential varies in space at hematite/water interfaces [75]. They determined how these quantities can show a substantial drop at the interface, which shows how the fields manipulate the mean inner potential of both the interfacial water and of the hematite itself, to further render the hematite more susceptible towards charge-carrier separation [75]. Therefore, the scope of external fields to alter and manipulate intrinsic, built-in electric fields is a yet further, more subtle point, emphasizing the whole concept of "field engineering", and the interplay between external Further, Futera and English performed NE-AIMD simulations on DC electric fields applied to similar hematite Fe 2 O 3 (001)/water interfaces [75].…”